1. Field of the Invention
This invention generally relates to filtering scratches out of defect detection results of wafer inspection.
2. Description of the Related Art
The following description and examples are not admitted to be prior art by virtue of their inclusion in this section.
Inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield in the manufacturing process and thus higher profits. Inspection has always been an important part of fabricating semiconductor devices such as ICs. However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the devices to fail.
Inspection processes generally detect a significant amount of potential defects that are actually nuisance defects on the wafer or noise. Nuisance defects as that term is generally used in the art can be defined as defects that are detected on the wafer that the user does not care about or events that are detected as defects but are not any kind of actual defects. For instance, in some wafer inspections, defects that are located on layers of the wafer under the layer being inspected can be detected, but those defects on underlying layers may not be of interest for the layer being inspected. In one such instance, scratches from a chemical mechanical polishing (CMP) layer can appear as optically real defects in inspections of later layers. However, scratches may not be defects of interest for the layer being inspected. Therefore, scratches on one layer can cause the nuisance rate of inspection of later layers on a wafer to be higher. As such, detecting such nuisance defects and reporting them as actual defects to a user have a number of disadvantages such as obscuring actual defects that a user cares about and/or affect the device fabrication in a meaningful manner in the inspection results, obstructing process control attempts to correct the actual defects that a user cares about, and causing users to change the wafer inspection process in ways to reduce the detection of nuisance defects that cause fewer defects of interest to be detected.
Since nuisance defects will be detected by most every inspection process, many inspection processes have been developed in which defect detection is performed and then the detected defects are filtered in some manner to separate defects of interest from nuisance defects. For instance, when there may be previous layer scratches detected in an inspection process, the inspection process may include using size or shape features to filter out relatively large scratches. Such filtering may be effective for some scratches but perhaps not all scratches. For instance, scratches caused by a CMP process include micro-scratches, which can have a substantially faint tail. Regular inspection processes may not detect the relatively faint tail of the micro-scratches. Therefore, only a portion of a micro-scratch having a relatively faint tail can appear in defect detection results thereby causing the micro-scratch to appear to have a shape in an image of the wafer that is different from its actual shape on the wafer. As such, defect filtering that is performed based on the shape of defects may not classify such micro-scratches as scratch defects. Consequently, the wafer inspection results may include a significant number of micro-scratches. In addition, since 60% to 70% of scratches caused by CMP processes can be micro-scratches having relatively faint tails, the wafer inspection results may include a significant percentage of all of the scratches on the wafer.
Accordingly, it would be advantageous to develop methods and systems for filtering defects detected on wafers that can be used to more effectively reduce the nuisance rate of inspection processes for wafers due to micro-scratches on the wafers.